1. Field of the Invention
The present invention relates to a recording/reproducing apparatus such as a digital video tape recorder (VTR) for recording/reproducing encoded data such as highly efficiently encoded image data.
2. Related Background Art
Digital processing of image data has been studied intensively. Many standardization methods for high efficiency encoding with image data compression have been proposed. High efficiency encoding technology encodes image data at a smaller bit rate in order to improve the efficiencies of digital transmission and recording. As such, high efficiency encoding schema, CCITT (Comito Consultafit International Telegraphique et Telephonique) has proposed Standardization Recommendation H.261 for Television Conferences/Picture Phones, JPEG (Joint Photographic Expert Group) scheme for color still images, and MPEG (Moving Picture Expert Group) for moving images. In the U.S.A., ATV (Advanced Television) using MPEG-2 scheme for a next generation TV broadcast system has been studied.
FIG. 1 is a diagram illustrating the MPEG coding system. In FIG. 1, arrows indicate the prediction direction in coding. FIG. 2 illustrates the order of image data in an encoding process, data disposed in a recording medium, a decoding process, and an image reproduction process, respectively of the MPEG coding system.
Referring to FIG. 1, in the MPEG coding system, a GOP (group of pictures) is constituted by a predetermined number of frame images. A GOP has at least one intra-frame encoded image I which is one-frame-image data encoded by DCT (Discrete Cosine Transform). One-frame image data at every a-th frames from the intra-frame encoded image I is converted into forward prediction encoded images P.sub.1 and P.sub.2 The forward prediction encoded image P.sub.2 is converted from the forward prediction encoded image P.sub.1. Each frame image data among the intra-frame encoded image I, first forward prediction encoded image P.sub.1, and second prediction encoded image P.sub.2 is converted into a forward/backward prediction encoded image obtained through forward/backward prediction encoding using the forward/backward image data.
As shown in FIGS. 1 and 2, first, the intra-frame image I is encoded. The intra-frame image I is encoded by using only information contained in one frame, and does not contain prediction in a time axis. Next, the forward prediction encoded image P is formed, and after the intra-frame encoded image I or forward prediction encoded image P is formed, an encoding process for the forward/backward prediction image B is performed. The forward prediction encoded image P and forward/backward prediction encoded image B use correlation with other image data. Because of the different prediction methods for respective image data described above, the forward/backward prediction encoded image B is recorded on a recording medium after the intra-frame encoded image I or forward prediction encoded image P, and it is moved to the original position when decoding.
Since the intra-frame image I is encoded only by the information in one frame, it can be decoded only by independent encoded data. On the other hand, the forward prediction encoded image P and forward/backward prediction encoded image B are decoded by using correlation with other image data, and cannot be decoded only by their independent encoded data.
In the MPEG coding system, although the record rate is stipulated (standard 1.2 Mbps), the data length is variable. Therefore, it is not possible to identify on what position the intra-frame encoded image I contained in GOP is recorded, nor to identify the data length of one GOP. If the MPEG coding system is applied to a digital VTR or other apparatuses, there is no problem in a normal reproduction mode because the encoded images I, B, and P are sequentially reproduced.
However, in a specific reproduction mode such as fast reproduction for a search, each encoded image I, B, P is not necessarily reproduced in a sequential order. Also in the specific reproduction mode, only part of a record track of a recording medium is reproduced and the position of the intra-frame encoded image I on the recording medium cannot be identified. Therefore, the intra-frame encoded image P cannot be always reproduced. In this case, the forward prediction encoded image P and forward/backward prediction encoded image B cannot be reproduced. A technique for solving this problem has been disclosed wherein encoded image data is recorded on a recording medium at the position allowing the reproduction thereof during the specific reproduction mode, and the encoded image data at that position is reproduced in the specific reproduction mode to obtain a frame image.
In the MPEG-2 coding system, coded image signals; voice signals, or other bit trains are called an elementary stream. A PES (Packetized Elementary Stream) packet for the unit structure of transforming the elementary stream is defined by the MPEG-2 coding system. This structure has a PES payload followed by PES header information. In the MPEG-2 coding system, a collection of elementary streams having a common time base is called a program.
In the MPEG-2 coding system, two formats are defined, one being a transport stream and the other being a program stream. Data necessary and sufficient for decode/reproduction synchronizations of images and voices are contained in both the definitions of the transport stream and program stream. The program stream is one or more PES packets having a common time base, and integrated into a single bit train.
The transport stream is one or more independent programs combined into a single bit train, and each program can have an independent time base. In the ATV system, the transport stream is used.
In the decoding system of the MPEG-2 coding system, synchronization is ensured by using an SCR (System Clock Reference) for the program stream and a PCR (Program Clock Reference) for the transport stream.
FIG. 3 is a block diagram showing an example of a decoder circuit for SCR or PCR. In FIG. 3, reference numeral 1 represents an input terminal, reference numeral 2 represents a substractor, reference numeral 3 represents a LPF (Low Pass Filter), reference numeral 4 represents an amplifier, reference numeral 5 represents a VCO (Voltage Controlled Oscillator), reference numeral 6 represents a counter, and a reference numeral 7 represents an output terminal.
The operation of this circuit will be described.
An output of the counter 6 is called an STC (System Time Clock). An SCR or PCR applied to the input terminal 1 of this decoder circuit is compared with the current STC outputted from the counter 6 at the subtractor 2. The comparison value passes through LPF 3 and the amplifier 4 which outputs it as a control signal of VCO 5. VCO 5 outputs a clock signal which changes its frequency in accordance with the control signal.
An output signal of VCO 5 at the output terminal 7 is used as a system clock and inputted to an MPEG decoder (not shown) and to the counter 6. The counter 6 counts this system clock to supply a new STC to the subtractor 2 to thereby constitute a feedback loop or a PLL (Phase Locked Loop). A time interval of SCR or PCR inputted to PLL is set to 700 msec or shorter for SCR and 100 msec or shorter for PCR.
In the ATV system, PCR is used for synchronization similar to the decoder circuit. In the case of a transport stream of the MPEG-2 coding system, image data, voice data, and other data are divided into a fixed length transmission unit called a transport packet and transported as a bit stream. This bit stream is added with PCR and other identifier information called a PSI (Program Specific Information) when necessary, and the decoder circuit detects and reproduces this PSI to initialize the decoder and correctly decode the bit stream.
PSI contains information for discriminating PIDs (Packet IDs) called a PAT (Program Association Table) and a PMT (Program Map Table). By using this information, a transport packet having target data is discriminated.
In transitting the specific reproduction mode of a VTR to the normal reproduction mode, it is necessary to again initialize the decoder circuit by using PCR and PSI for the normal reproduction different from PCR and PSI for the specific reproduction, because the image data for the normal and specific reproductions are independently and time sequentially written on the recording medium. Furthermore, since PCR becomes discontinuous between the normal and specific reproductions, it is necessary to set the discontinuity indication in the header of the packet to "1" to inform the decoder circuit of the PCR discontinuity.
FIG. 4 illustrates the procedure of decoding a bit stream in the ATV system. As shown in FIG. 4, a transport packet recorded with PAT, i.e., a transport packet having PID of 0.times.0000 recorded in the transport header is detected from the bit stream of the ATV system. PAT is picked up from the payload of the transport packet having PID of 0.times.0000, and a program map PID is read from this PAT. The program map PID is a PID of a transport packet in which a PID (elementary PID) of the transport packet containing a program to be decoded is described.
Next, a transport packet having the same PID as the program map PID is detected. A PMT is picked up from the payload of the detected transport packet. From this PMT, an elementary PID is read which is a PID of the transport packet recorded with the desired program. A transport packet having the same PID as the elementary PID is detected. The desired program is read from the payload of the transport packet having the PID same as the elementary PID.
In the conventional technologies described above, in transitting from the specific reproduction mode such as a forward search to the normal reproduction mode, it is impossible for an ATV decoder to decode a bit stream correctly until VTR reproduces PSI of the normal reproduction.
FIG. 5 illustrates an example a bit train reproduced from a recording medium in the unit of transport packet. Packets with hatched lines are those for the normal reproduction, and the other packets are those for the specific reproduction. During the normal reproduction, PES and PSI packets for the normal reproduction (1) and PES and PSI packets for the specific reproduction are reproduced. When a mode transition signal for the transition from the normal reproduction mode to the specific reproduction mode such as a search is given at the timing indicated by an arrow in FIG. 5, the transport system of the recording medium starts transitting to the specific reproduction mode, and enters a data reproduction disabled state as indicated at (2) until data are stabilized. After the state is stabilized, the data for specific reproduction starts being reproduced. However, in this case, the decoder cannot discriminate the PES packet for specific reproduction until the PSI packet for specific reproduction indicated at (4) is reproduced. Therefore, the reproduction is possible only from the packet indicated at (5), and the data decoding disabled period indicated at (3) is generated so that a reproduced image is disturbed or lost.
The time period from when the specific reproduction such as a forward search transits to the normal reproduction mode to when PSI is first detected, is not necessarily constant. In transitting to the normal reproduction mode after the search end, the time period (transition time) from a search screen to a normal reproduction screen is not constant depending upon the transition timing, and during this time period, the image is disturbed or lost.
Since the MPEG coding system or ATV system uses PCR or SCR for synchronization, it is necessary to continuously input PCR or SCR to the decoder. However, in the case of the above-described recording/reproducing apparatus, during the transition period from the normal reproduction to specific reproduction or vice versa, PCR or SCR cannot be inputted to the decoder so that the operation of the decoder is delayed.